Process for the manufacture of refractory ceramic products

ABSTRACT

A ceramic molding composition comprising an alcoholate of aluminum or a metal of the fourth sub-group of the Periodic System, a refractory metal oxide and an agent selected from the group consisting of diketones of the formula ##STR1## wherein R 1  and R 2  are each independently an aliphatic radical of between C 1  and C 18  carbon atoms or an aromatic radical from C 6  to C 18  carbon atoms, a hydroxycarboxylic acid compound of the formula ##STR2## wherein R 3  is an aliphatic radical of C 1  and C 18  or an aromatic radical from C 6  to C 18 , and R 4  is hydrogen or an alkyl radical of C 1  to C 4 , and a polyvalent alcohol, an improvement in the formation of a ceramic product wherein a ceramic molding composition is shaped and hardened, which improvement resides in employing the above defined ceramic molding composition.

This is a continuation of application Ser. No. 326,624 filed Jan. 26,1973 now abandoned.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The invention relates to a process for manufacturing refractory ceramicproducts such as molds and cores for metal casting. The products made bythe process of the invention are characterized by an especially highability to withstand aggressive molten metals.

2. DISCUSSION OF THE PRIOR ART

It is known that ceramic molds are made by mixing a finely dividedrefractory material with a usually siliceous binding agent. By suchprior art methods the refractory particles are mixed with the bindingagent and applied to a model by immersion, spraying or pouring. Suchmolds are hardened by drying or by the use of a liquid, solid or gaseoushardener such as ammonia, the liquid binding agent being transformed toa gel and the particles of refractory material being cemented together.When siliceous binding agents are used, a silica gel forms, which istransformed to silicon dioxide in the firing operation that follows.

Molds and cores made by these known methods, however, do not alwaysproduce satisfactory results. Particularly in the casting of types ofsteel having a relatively high carbon content or types having a highchromium content, reactions often occur between the molten metal and themold, so that the castings have flawed surfaces.

The use of aluminum alcoholates or alcoholates of metals of the fourthsub-group of the Periodic System in the prior art methods used for theproduction of ceramic molds has hitherto been impossible owing to thegreat sensitivity of these compounds to moisture. These alcoholatescondense partially, as a result of hydrolysis, with the atmosphericmoisture to form products of higher molecular weight which can no longerbe used as binding agents. The end product of the condensation reactionis always the corresponding oxide.

It has, therefore, become desirable to provide a ceramic moldingcomposition which can readily be formed to a desired shape and hardened,which ceramic composition is relatively insensitive to water. Moreover,it has become desirable to provide a ceramic molding composition whichis substantially free of silica or siliceous materials so that theresultant hardened ceramic object is free from surface defects.

SUMMARY OF THE INVENTION

Broadly speaking, this invention contemplates a ceramic moldingcomposition comprising:

A. An alcoholate of aluminum or metal of the fourth sub-group of thePeriodic System;

B. A refractory metal oxide; and

C. An agent selected from the group consisting of:

(1) a beta-diketone of the formula ##STR3## wherein R₁ and R₂ are eachindependently an aliphatic radical of between C₁ and C₁₈ carbon atoms,substituted or unsubstituted, or an aromatic radical of between C₆ andC₁₈, either substituted or unsubstituted;

(2) a hydroxycarboxylic acid compound of the formula ##STR4## wherein R₃is an aliphatic radical of C₁ to C₁₈ carbon atoms or an aromatic radicalof C₆ to C₁₈ carbon atoms, either substituted or unsubstituted, and R₄is hydrogen or a substituted or unsubstituted alkyl radical of C₁ to C₄carbon atoms; and

(3) a polyvalent alcohol.

As indicated above, the present invention is also directed to animprovement in the process for the manufacture of ceramic productswherein a ceramic molding composition is shaped about a form and afterbeing shaped, it is hardened, which improvement comprises employing asthe ceramic molding composition the above-defined composition, includingone of the agents specified under paragraph C.

It has been found, pursuant to the invention, that beta-diketones,hydroxycarboxylic acids or their lower alkyl esters or polyvalentalcohols render ceramic molding compositions containing alcoholates ofaluminum or group 4 metals relatively water insensitive so that thecomposition can be free of silica and can be resistant to the adverseeffects of some materials which may come in contact with the surface ofthe finished ceramic object, e.g., carbon, chromium and titanium.

The beta-diketones of the present invention generally have the aboveformula in which R₁ and R₂ are each independently an aromatic radical oran aliphatic radical, either substituted or unsubstituted. Preferably,R₁ and R₂ are alkyl radicals, especially of C₁ to C₈ carbon atoms, e.g.,methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl and hexyl.They can also be aryl radicals, such as phenyl, naphthyl, anthracyl andphenanthryl.

Instead of employing a beta-diketone, the ceramic composition cancontain, as the agent which renders the composition less watersensitive, a hydroxycarboxylic acid or a lower alkyl ester thereof.Generally speaking, the hydroxycarboxylic compound will have the formulaexpressed in C.(2) above. R₄ will be hydrogen or an alkyl radicalgenerally of C₁ to C₄ carbon atoms. R₃, on the other hand, can be alkyl,cycloalkyl, alkenyl, aralkyl, or heterocyclic. If straight chained, R₃can have C₁ to C₁₈ carbon atoms, especially C₁ to C₈. If cyclic, it cansuitably have between 5 and 7 members in the ring, especially between 5and 6. Additionally, R₃ can be an aryl group of C₁ to C₁₈ carbon atoms.It should be understood that hydroxyl substitution on the R₃ moiety neednot be on the alpha carbon atom of the acid. Indeed, the hydroxyl groupcan be on any R₃ carbon atom, no matter how remote from the acid group.

As a third alternative, a polyvalent alcohol can be employed in theceramic molding composition, which polyvalent alcohol is preferably apolyvalent alcohol of an alkane, an alkene or an alkyne.

Alternatively, aromatic alcohols, especially phenols, can be employed.The hydroxyl groups of the polyvalent alcohol can be in any positionwith respect to one another. However, it is preferred that the alcoholbe a C₂ to C₁₂ alkanol.

The ceramic molding compositions of the present invention generally havea composition within the following range:

                  TABLE I                                                         ______________________________________                                        Component                                                                             Name              Broad    Preferred                                  ______________________________________                                                                  % by wt. % by wt.                                   A       Alcoholate of Aluminum                                                        or Group 4 Metal  9 to 45  20 to 25                                   B       Refractory Metal Oxide                                                                          50 to 90 72.5 to 78                                 C       Beta-Ketone; Hydroxy-                                                         carboxylic Acid or                                                            Ester; or Polyvalent                                                          Alcohol           1 to 5   2 to 2.5                                   ______________________________________                                    

Preferably, the refractory metal oxide has a particle size between 0.1and 2.0 microns.

DESCRIPTION OF SPECIFIC EMBODIMENTS

A process has now been found for the manufacture of ceramic productsthrough the mixing of refractory oxides with a binding agent, followedby shaping and hardening, which is characterized in that the bindingagent is a mixture of aluminum alcoholate and/or alcoholates of metalsof the fourth sub-group of the Periodic System formed withbeta-diketones and/or hydroxycarboxylic acids or their esters and/orpolyvalent alcohols.

The sensitivity to moisture of the binding agents of the invention is sodiminished by the addition of beta-diketones and/or hydroxycarboxylicacids or their esters and/or polyvalent alcohols that they can be usedin known processes instead of siliceous binding agents. Any slightcondensation that might occur, resulting in dimeric or trimericcondensation products, does not interfere with their use as bindingagents.

The binding agents of the invention are to be in liquid form at roomtemperature, insofar as possible. The metal alcoholate components arealcoholates of C₁ to C₈ carbon atoms, especially of alkanols, and arepreferably the propylates and butylates of the above-mentioned metals.However, those alcoholates of these metals may also be used whosealcohol component is an aliphatic alcohol of 1 to 8 carbon atoms,providing the corresponding metal alcoholates are in liquid form at roomtemperature.

The second component present in the binding agent of the invention inaddition to the metal alcoholates should, insofar as possible, also be acompound that is liquid at room temperature. In many cases this secondcomponent reacts with the metal alcoholates to form chelates or othersuch complex compounds. Such a reaction product can also be added inpreviously finished form to a batch of binding agent.

The quantity of this second component is to be selected such that whenmixed with the metal alcoholate it does not result in the precipitationof solids in the binding agent. By varying these additives the speed ofthe hydrolysis of the binding agent may be adjusted as required. Amountsof between 2 and 25% of the weight of the metal alcoholate maypreferably be used, although the use of larger amounts is possible.

Both aliphatic and aromatic diketones or mixed aliphatic and aromaticdiketones may be used as beta-diketones. The following are examples:acetylacetone, acetic acid ester, benzoylacetone or dibenzoylmethane.

The hydroxycarboxylic acids which can be used as the second componentare preferably liquid aliphatic hydroxycarboxylic acids. These acids cancontain either one or a plurality of hydroxy groups. Examples areglycolic acid, lactic acid or oxypropionic acid, as well as alkyl estersthereof. The alkyl radical of the esters is preferably one having 1 to 4carbon atoms. Higher hydroxycarboxylic acids or their alkyl esters mayalso be used, examples ω-oxycapronic acid or ω-oxycaprylic acid ethylester.

The polyvalent alcohols are both bivalent and trivalent andhigher-valency alcohols. The position of the hydroxy groups in relationto one another is of no importance. Of the bivalent alcohols, theglycols with 2 to 12 carbon atoms are preferred, such as ethyleneglycol, 1,4-butanediol, hexylene glycol or octylene glycol; however,diglycols may be used, such as ethylene diglycol or propylene diglycol.Also usable as bivalent alcohols are the dialkanolamines, especiallydi(C₁ -C₅) alkanolamines such as diethanolamine. Examples of trivalentalcohols are glycerine and triethanolamine.

The binding agents of the invention can be used both directly anddissolved in a solvent. The addition of a solvent has the advantage thatin this manner the concentration of the alcoholates contained in thebinding agent can be adjusted, and hence, the amount of metal oxide thatis precipitated in the entire body after the hardening of the latter canbe controlled. The use of a solvent is recommendable also when thesecond component of the binding agent of the invention is not liquid atroom temperature, such as 1,8-octylene glycol, for example.

Alcohols, ketones, ethers and aliphatic and aromatic hydrocarbons aresuitable solvents. In the selection of the solvent, however, care mustbe taken to see that the solvent does not have the ability to dissolveany wax model that might be used in making the mold. Examples ofsuitable solvents are isopropyl alcohol, butanone, methylisobutylketone,dioxane, toluene, xylene and benzene. Generally, the binding agentcomprises the metal alcoholate and agent, e.g., beta-diketone is presentin the solution in an amount between 2 and 25% by weight.

The preparation of the molds with the binding agent of the invention isperformed in a conventional manner by mixing the binding agent with therefractory oxides. Since the molds made with these binding agents areattacked only lightly or not at all by high alloy types of steel, suchas carbon steel, or steels containing high percentages of chromium, oreven by aggressive metals, such as titanium, they are preferentiallysuitable as expendable molds in the casting of these metals. Therefractory oxides used in molds for these metals are to contain lessthan 10% by weight SiO₂, preferably no silica, since the latter canreact with the metals at the high temperatures prevailing during thecasting.

The refractory oxides which can be used with the aid of the bindingagent of the invention to make ceramic products with high resistance toaggressive molten metals therefore include preferably corundum,especially alpha alumina as well as other forms thereof; magnesia, TiO₂,ZrO₂, ThO₂, CaO, BeO and Cr₂ O₃.

The molding preparations made with the binding agent of the inventionmay also be combined with molding compounds which contain binding agentsof the prior art, for example, in the case of fine molds made frommodels that can be removed by melting them out, by making the first twolayers of the ceramic mold of a molding compound with the binding agentof the invention, the rest of the layers being made of molding compoundsusing binding agents of the prior art such as silicates.

In order to more fully illustrate the invention and the manner ofpracticing the same, the following Examples are presented:

EXAMPLE 1

100 parts by weight of aluminum-sec.-butylate are mixed with 30weight-parts of acetyl acetone and 30 weight-parts of butanone, a slightwarming taking place.

Then 320 weight-parts of fused corundum, finely ground, maximim grainsize 0.07 mm, are added. The mixture produces a dipping compound fromwhich any desired investment can be made by drying and hardening. Thenmodels of wax or polystyrene or urea or other such material are dippedand after dipping are sprinkled with finely granular corundum of a grainsize of 0.12 to 0.25 mm. After dipping and sanding, the coating ishardened in the air or it is sprayed for more rapid hardening with ahardener fluid of the prior art. The hardener fluid consists, forexample, of a mixture of 1.25% aqueous ammonia and ethyl alcohol in avolume ratio of 1:1.

After the coating has solidified, the dipping is repeated, and thesecond coating is hardened in the same manner. After the hardening ofthe second coat, additional coats are applied of materials which areused in known processes, such as fused mullite with hydrolyzed ethylsilicate as the binding agent. The layers of molding compound made withethyl silicate as the binding agent may also be hardened with theabove-described hardeners or with gaseous ammonia. After the coats arehardened the model is removed by melting, burning or dissolving, and thehollow form resulting is fired at temperatures of over 1000° C. Thefired mold is filled with metal while either cold or hot. It is suitablefor the casting of high-alloy steel at high casting temperatures.

EXAMPLE 2

100 weight-parts of zirconium-n-butylate are mixed with 10 weight-partsof acetyl acetone, and 275 weight-parts of fused, finely groundzirconium oxide (max. grain size 0.06 mm) are stirred into this mixture.

Two coats of this dipping compound are applied as described in Example 1and hardened as in Example 1. The other coats consist of the materialdescribed in Example 1. In this manner a hollow mold is obtained whichis suitable for the casting of metals which have a strong tendency toreact with the mold, such as carbon steel, high-percentage chromiumsteels and titanium.

EXAMPLE 3

Onto a divided mold of plastic, metal or wood, which has been treatedwith a parting agent, a suspension is sprayed which is composed of thefollowing substances: 85 weight-parts of zirconium-n-propylate, 15weight-parts of acetic acid ester and 250 weight-parts of zirconiumoxide, grain size 0 to 0.06 mm.

The spraying is performed by means of an air spray gun. The sprayedcoating is about 0.5 mm thick. Then the coating is dusted with a coarserzirconium oxide. After dusting, a hardener consisting of 1.25% aqueousammonia and alcohol is sprayed on. After the solidification of the primecoat, a second coat of the same material is applied, or a direct backingis applied with a ceramic compound consisting of a refractory materialand a binding agent on a basis of ethyl silicate containing a hardener.This compound is the same as the compounds which are used in the knownShaw process.

After the Shaw compound has set, the model is removed from the moldingcompound. After post-hardening, flame cleaning and then firing, the moldis ready for the casting. In the molds made by this method castings canbe made of high-alloy steels, of high cast weights and with flaw-freesurfaces.

EXAMPLE 4

100 weight-parts oftetraisopropyltitanate, 50 weight-parts oftriethanolamine and 45 weight-parts of isopropanol are mixed together,and fused magnesia is stirred into the mixture.

With this dipping compound a wax model is invested by dipping it twice.To accelerate hardening the model is sprayed with a water-alcoholmixture. After applying the first two coats, the model is covered withadditional ceramic layers containing ethyl silicate as binding agent.

What is claimed is:
 1. A ceramic molding composition which consistsessentially of:(1) a refractory metal oxide; and (2) a non-siliceousbinding agent which is in liquid form at room temperature, said bindingagent consisting essentially of:(A) an uncondensed metal alcoholate ofaluminum or a metal of the Fourth Sub-Group of the Periodic System,which alcoholate is in liquid form at room temperature; and (B) an agentwhich is normally liquid at room temperature which agent is selectedfrom the group consisting of:I. a β-diketone of the formula ##STR5##wherein R₁ and R₂ are each independently a substituted or unsubstitutedaliphatic radical of C₁ to C₁₈ carbon atoms or a substituted orunsubstituted aromatic radical of C₆ to C₁₈ carbon atoms and Ii. ahydroxycarboxylic acid compound of the formula ##STR6## wherein R₃ is asubstituted or unsubstituted aliphatic radical of C₁ to C₁₈ carbon atomsor an aromatic radical of C₆ to C₁₈ carbon atoms, and R₄ is hydrogen ora lower alkyl group; said agent present in an amount between 2 and 25%by weight of the metal alcoholates and in such amount that when inadmixture with said metal alcoholate precipitation of solids does notoccur, said composition substantially free of silica or siliceousmaterial.
 2. A ceramic molding composition according to claim 1, whereinsaid agent is a hydroxycarboxylic acid.
 3. A process for the manufactureof a ceramic mold which comprises applying a ceramic molding compositionto a model and shaping said ceramic molding composition about saidmodel, and thereafter hardening said ceramic molding composition on saidmold, said ceramic molding composition being substantially free ofsilicon and consisting essentially of:(1) a refractory metal oxide; and(2) a non-siliceous binding agent which is in liquid form at roomtemperature, said binding agent consisting essentially of:(A) anuncondensed metal alcoholate of aluminum or a metal of the FourthSub-Group of the Periodic System, which alcoholate is in liquid form atroom temperature; and (B) an agent which is normally liquid at roomtemperature which agent is selected from the group consisting of:I. aβ-diketone of the formula ##STR7## wherein R₁ and R₂ are eachindependently a substituted or unsubstituted aliphatic radical of C₁ toC₁₈ carbon atoms or a substituted or unsubstituted aromatic radical ofC₆ to C₁₈ carbon atoms and Ii. a hydroxycarboxylic acid compound of theformula ##STR8## wherein R₃ is a substituted or unsubstituted aliphaticradical of C₁ to C₁₈ carbon atoms or an aromatic radical of C₆ to C₁₈carbon atoms, and R₄ is hydrogen or a lower alkyl group; said agentpresent in an amount between 2 and 25% by weight of the metal alcoholateand in such amount that when in admixture with said metal alcoholateprecipitation of solids does not occur, said composition substantiallyfree of silica or siliceous material.
 4. A process according to claim 3wherein after said ceramic molding composition is hardened on saidmodel, said model is removed.
 5. A process according to claim 4 whereinsaid model is removed by melting, burning or dissolving.
 6. A processaccording to claim 4 wherein said alcoholate of aluminum or a metal ofthe Fourth Sub-Group of the Periodic System is an alcoholate of analiphatic alcohol of 1 to 8 carbon atoms, said refractory oxide isselected from the group consisting of alumina, titania, zirconia,thoria, calcium oxide, BeO and chromia, and said components are presentin the following ratios:Alcoholate of aluminum or Group IV metal -- 20to 25% by weight Refractory metal oxide -- 78% by weight β-ketone;hydroxycarboxylic acid or ester thereof -- 2 to 2.5% by weight.
 7. Aprocess according to claim 4 wherein after an initial coating is appliedto said model and hardened, the same is recoated on at least one sidethereof with a ceramic refractory metal oxide composition which isthereafter hardened.
 8. A process according to claim 7 wherein thecoating so formed is treated with additional ceramic refractory oxidecomposition comprising fused mullite and a partially hydrolyzed ethylsilicate, the ethyl silicate being hydrolyzed to between 1 and 50%, andthe same is thereafter hardened with a hardener comprising ammonia orethyl alcohol or a mixture thereof.
 9. A process according to claim 4wherein the ceramic composition is applied to said model by beingsprayed thereon.
 10. A process according to claim 4 wherein said ceramicmolding composition is free of silica or a siliceous material, saidalcoholate, refractory metal oxide and β-ketone, hydroxycarboxylic acidor ester are present in the ceramic molding composition in the followingrange:Alcoholate of aluminum or Group IV metal -- 9 to 45% by weightRefractory metal oxide -- 50 to 90% by weight β-ketone;hydroxycarboxylic acid or ester - 1 to 5% by weight.
 11. A processaccording to claim 4 wherein the ceramic molding composition contains asolvent selected from the group consisting of isopropyl alcohol,butenone, methylisobutylketone, dioxane, toluene, xylene and benzene.12. A process according to claim 4 wherein said ceramic moldingcomposition is free of silica or a siliceous material, said agent is aβ-diketone and the alcoholate, refractory metal oxide and β-diketone arepresent in the ceramic molding composition in the followingrange:Alcoholate of aluminum or Group IV metal -- 9 to 45% by weightRefractory metal oxide -- 50 to 90% by weight β-diketone -- 1 to 5% byweight.
 13. A process according to claim 12 wherein the β-diketone isselected from the group consisting of acetyl acetone, acetic acid ester,benzoyl acetone and dibenzoyl methane.
 14. A process according to claim4 wherein said ceramic molding composition is free of silica or asiliceous material, said agent is a hydroxycarboxylic acid or ester andthe alcoholate, refractory metal oxide and hydroxycarboxylic acid orester are present in the ceramic molding composition in the followingrange:Alcoholate of aluminum or Group IV metal -- 9-45% by weightRefractory metal oxide -- 50-90% by weight Hydroxycarboxylic acid orester -- 1-5% by weight.
 15. A process according to claim 14 wherein thehydroxycarboxylic acid or ester is glycolic acid, lactic acid,oxypropionic acid, ω-oxycapronic acid or ω-oxycaprylic or their alkylesters.